1. Field of the Invention:
This invention relates to a motor driven photographing control circuit which controls a double preference type automatic exposure control camera used in combination with a motor drive device for continuous photographing.
2. Description of the Prior Art:
Among single-lens reflex cameras of advanced types, TTL-AE cameras which are equipped with automatic exposure control devices of so-called TTL systems are used for various purposes. The cameras of this kind are divided into two types. One is a shutter preference type wherein the shutter speed is first set and then the optimum stop value is automatically obtained. The other is a stop preference type wherein the stop value is first decided and then the shutter speed is automatically obtained. The advantage of one type over the other depends upon the photographing conditions.
Therefore, for such TTL-AE cameras, it is considered most desirable to have a type that permits photographing in both the shutter preference mode and the stop preference mode with provision for selection between the two modes. For such a double preference mode, there have been disclosed various methods. On the other hand, motor driven AE cameras which are of such a double preference type and permit attachment of motor drive devices thereto have been developed to meet demands, which have increased particularly among news photographers and high class amateur photographers.
Heretofore, the motor driven cameras of this type have been generally prepared by attaching a motor drive unit to an AE camera of a shutter preference type. With such a MD (motor driven) camera, when continuous photographing is carried out by presetting the shutter speed at a given value, the time per frame of film in continuous photographing becomes almost constant. Whereas, when continuous photographing is carried out with a motor drive device attached to an AE camera of a stop preference type, the shutter speed is automatically changed by the AE (automatic exposure control) device according to the brightness of the object being photographed. By this, the number of frames of film per second in the continuous photographing accomplished with the motor drive device also varies. This causes some inconvenience.
In order to make continuous photographing in short cycles with a motor drive device attached to an automatic exposure controlled camera, when a first film frame has been exposed, the film is immediately wound up in response to an exposure completion signal and a shutter charging action is accomplished. Then, in responce to a winding completion signal, shutter is released from the charged state for carrying out photographing on a second film frame. The cycle Tc in such continuous photographing is expressed by the following formula: EQU Tc .apprxeq. TE + TW (1)
wherein TE indicates shutter time (exposure time). In a focal plane single-lens reflex camera, the shutter time is a length of time from shutter release to completion of the travel of a rear diaphragm. TW indicates a length of time required for film winding and shutter charging by a motor drive device. In Formula (1), when the winding time TW is about constant, the photographing cycle Tc will be also about constant in a shutter preference camera, because the preset shutter time is deemed to remain constant during a continuous photographing operation. However, in the case of a stop preference camera, the shutter time is determined by the automatic exposure control action of the camera according to the brightness of the object being photographed. With a stop preference camera, therefore, TE in Formula (1) varies and the continuous photographing cycle Tc also varies. To solve the problem of varying Tc in the stop preference mode, there is a method wherein a release timer is provided to begin counting concurrently with shutter release and a film winding operation is started by an output of an AND gate which is provided for a count completion signal of the release timer and an exposure completion signal.
Assuming that the continuous photographing cycle in accordance with this method is Tc' and the time controlled by a release timer is TR, when TR is longer than the exposure time TE, the cycle Tc' is expressed by: EQU Tc' .apprxeq. TR + TW (2)
in Formula (2) above, when the time TR of the release timer is set at a constant value, the photographing cycle Tc' becomes about constant. However, the winding-up time TW is not always constant but varies with variation in the torque of film winding, etc. Accordingly, it has been difficult to make the photographing cycle perfectly constant even by the above described method. Furthermore, such variation in Tc due to variation in TW takes place also in the case of a shutter preference mode. Therefore, the same difficulty has been experienced also in the shutter preference mode.
For the stop preference mode, there has been also disclosed another method wherein: In addition to the timer which controls the shutter time of the camera, there is provided an interval timer which determines the length of time between shutter release and the next shutter release, i.e. a photographing cycle; and a shutter release signal for the next photographing operation is produced by the output of an AND gate between a winding completion signal and a count completion signal of the interval timer. By this arrangement, even when winding is not completed within the preset cycle due to a prolonged shutter time, the next photographing operation can be smoothly started immediately after completion of winding. In this method, however, when the signal from the interval timer which determines the photographing cycle is supplied to the AND gate prior to the winding completion signal, the cycle TI of the interval timer becomes TI &lt; TR + TW. Therefore, the continuous photographing cycle Tc becomes slower than the preset cycle TI of the interval timer.